1. Field of the Invention
The present invention relates to a multi-wavelength semiconductor laser, and more particularly, to a fabrication method of a multi-wavelength semiconductor laser capable of emitting laser beams of different wavelengths simultaneously or selectively.
2. Description of the Related Art
In general, a semiconductor laser is a semiconductor device outputting light amplified by induced emission, with the emitted light having a narrow frequency bandwidth, excellent directivity, and guaranteed high output. Because of these merits, it is popular as a light source for the optical pick-up devices for optical disc systems such as a Compact Disc (CD) or a Digital Video Disc (DVD).
Recently, the field of optical disc technology requires a multi-wavelength semiconductor laser device capable of oscillating two or more of different wavelengths. The most representative example is a two-wavelength semiconductor laser used for the relatively low-density CD player (780 nm) and the relatively high-density DVD player (635 nm or 650 nm).
FIGS. 1a through 1f are process flow diagrams illustrating a fabrication method of a conventional two-wavelength semiconductor laser device. More specifically, these diagrams illustrate the fabrication method of a two-wavelength semiconductor laser device in which the AlGaAs-based first semiconductor laser (780 nm wavelength light) and the AlGaInP-based second semiconductor laser (650 nm wavelength light) are provided monolithically on a single substrate.
First, as shown in FIG. 1a, epitaxial layers for a first semiconductor laser are formed on an n-type GaAs substrate 11. That is, an n-type AlGaAs cladding layer 13a, an AlGaAs-based active layer 14a, a p-type AlGaAs cladding layer 15a, and a p-type cap layer 16a are grown in their order.
Thereafter, as shown in FIG. 1b, the epitaxial layers 13a, 14a, 15a, and 16a are removed selectively to expose an area on the surface of the GaAs substrate 11 via photolithography and etching processes.
Thereafter, as shown in FIG. 1c, epitaxial layers for a second semiconductor laser are formed on the exposed surface of the GaAs substrate 11. That is, the n-type AlGaInP cladding layer 13b, the AlGaInP-based active layer 14b, the p-type AlGaInP cladding layer 15b, and the p-type cap layer 16b are grown in their order.
Thereafter, as shown in FIG. 1d, epitaxial layer 13b, 14b, 15b, and 16b portions of the second semiconductor laser on the epitaxial layers 13a, 14a, 15a, and 16a of the first semiconductor laser are removed via additional photolithography and etching processes, and simultaneously, the remaining epitaxial structure of the second semiconductor laser is separated from that of the first semiconductor laser.
Next, as shown in FIG. 1e, the p-type AlGaAs cladding layer 15a and the p-type AlGaInP cladding layer 15b are selectively etched by the conventional method to form ridge structures to improve the current injection efficiency.
Lastly, as shown in FIG. 1f, current limiting layers 18a and 18b are formed on the p-type cladding layers 15a and 15b each with the ridge structure formed thereon, and then, each p-type cap layer is exposed via photolithography and etching processes. Then, p-side electrodes 19a and 19b are formed with Ti, Pt and Au and alloys thereof on the p-type cap layers 16a and 16b, and an n-side electrode 19c is formed with Au/Ge, Au, Ni and alloys thereof on the underside of the GaAs substrate 11.
As explained in the above process, semiconductor lasers 10a and 10b with two different wavelengths can be formed on the same substrate 11 to provide a two-wavelength laser device 10 in a single chip form.
However, in the conventional fabrication method of a two-wavelength semiconductor laser, not only photolithography and etching processes have to be repeated for so many times but also the etching process for separation of laser devices shown in FIG. 1d requires complex etching conditions, complicating the entire processes, which results in a lower yield.
Particularly, since high selectivity of the AlGaAs-based epitaxial layer of the first semiconductor laser and the GaAs substrate is not guaranteed, the surface of the substrate on which the second semiconductor is to be subsequently grown after the etching process in FIG. 1b may easily be damaged, and in some severe cases, part of the substrate may be etched in a certain depth. Therefore, in the conventional method, it may be difficult to obtain quality epitaxial layers, and also may cause inadequate alignment in which the active layers of the two lasers may be vertically misaligned with certain size of gap.